1. Field of the Invention
The present invention relates to a process for forming a gel having improved creep resistance at elevated temperatures and having a viscosity of at least about 100,000 cps which includes the steps forming a solvent system of an organic liquid and a water miscible cosolvent, the water miscible cosolvent being less than about 15 wt. % of the solvent system, a viscosity of the solvent system being less than about 1000 cps; dissolving a neutralized sulfonated polymer in the solvent system to form a solution, adding a free radical generating initiator to the solution at a concentration level of about 0.01 to about 1 parts by weight per 100 parts of organic liquid, a concentration of the neutralized sulfonated polymer in the solution being about 0.5 to about 20 wt. %, a viscosity of the solution being less than about 20,000 cps; and adding under high shear conditions about 5 to about 500 vol. % water to the solution having a viscosity less than about 20,000 cps, the water being immiscible with the solution and the water miscible cosolvent transferring from the solution phase to the water phase thereby causing the viscosity of said solution to increase from less than 20,000 cps to greater than 100,000 cps.
2. Description of the Prior Art
There are many applications for very viscous or gelled solutions of polymers in organic liquids which are quite diverse. There are also a number of physical and chemical techniques for preparing such systems. The present invention is concerned with a process for converting a relatively low viscosity organic liquid solution of an ionic polymer into a very viscous or gelled system via a rapid process which under certain conditions can be reversed. The potential applications for this process and the products derived therefrom will be evident in the instant application.
There are major problems in the direct preparation of viscous polymer solutions or gels via conventional techniques such as polymer dissolution. For example, attempts to form a high viscosity (&gt;500,000 cps) solution of polystyrene in a suitable solvent such as xylene can be difficult. The levels of polymer required are either very high (20 to 50 wt. % concentration) or the molecular weight of the polymer must be extremely high. In either event, the dissolution process is extremely slow even at elevated temperatures, and even then it is difficult to achieve homogeneous polymer solutions free of local concentrations of undissolved, or poorly dissolved polymer. Thus, the process of achieving such solutions can be difficult and the concentration of polymer in the solution to achieve high viscosities can be uneconomically high.
A number of U.S. patent applications have recently described the formation of gels of sulfonated polymers, but these gels display inferior creep resistance and differ from the instant invention in that they do not employ a free radical generating initiator. These U.S. patent applications are: Ser. Nos. 930,044; 106,027; 136,834; 136,835; 136,836; and 136,837.
One of the problems that has been observed with gels achieved by the routes described above is that at low sulfonate contents, the resulting gels display poor creep resistance over long periods of time and at elevated temperatures. This flow behavior is what would be predicted when one employs a polymer of low sulfonate content, employed at low concentrations, and activated with water. The number of physical crosslinks which are present are insufficient to sustain permanent crosslinking and flow occurs. This problem is especially severe at elevated temperatures (&gt;100.degree. C.). These observations are consistent with continued flow of ionomer gels under stress, especially at high temperature.
This invention describes an approach to create ionomer gels of radically improved stability both at high temperature and over long periods of time. This invention embodies a combination of a free radical-generating initiator in solution, preferably with the ionic polymer plus the use of a water miscible cosolvent. This entire combination is interacted with an aqueous solution accompanied by shear. The resulting mixture sets-up either to a more viscous solution or a solid gel. In the absence of free-radical initiators, gels occur as previously described. In the presence of benzoyl peroxide it has been observed that the gelation occurs much more rapidly at much lower water contents, resulting in an extremely creep resistant solid gel. The resulting solid appears to resist stress to an unexpected degree. The surprising observation in this invention is that the combination of peroxide and ionomer solution is activated by the addition of water at very low temperatures, substantially lower than would have been anticipated based on our knowledge of peroxide decomposition temperatures.
There appears to be little doubt that the peroxide is activating a crosslinking mechanism which is synergistically interacting with the ionic crosslinking normally expected. Irrespective of the mechanism for this process it is apparent that the resulting gelation displays the highly coveted characteristics of extreme creep resistance.
There are various chemical approaches to the solution of the problems outlined above, that is polymer chain lengthening reactions which can occur to give viscous solutions such as by the reaction of hydroxyl terminated polymers with diisocyanates etc. Such processes have inherent disadvantages which preclude their use in the intended applications of this invention.
The instant invention describes a process which permits (1) the preparation of polymer solutions of sulfonated polymers in organic liquid having reasonably low viscosities (i.e., less than about 20,000 cps); and (2) the preparation of gels from such solutions by the simple process of mixing water with the polymer solution. These operations are achieved by the use of the appropriate concentration of polymers having low concentrations of ionic groups present, preferably metal sulfonate groups. Such polymers are described in detail in a number of U.S. patents (U.S. Pat. Nos. 3,836,511; 3,870,841; 3,847,854; 3,642,728; 3,921,021) which are herein incorporated by reference. These polymers possess unusual solution characteristics some of which are described in U.S. Pat. No. 3,931,021. Specifically, such polymers such as lightly sulfonated polystyrene containing about 2 mole % sodium sulfonate pendant to the aromatic groups are typically not soluble in solvents commonly employed for polystyrene itself. However, the incorporation of modest levels of polar cosolvents permit the rapid dissolution of such ionic polymers to form homogeneous solutions of moderate viscosity.
In the instant process, the role of the polar cosolvent is that of solvating the ionic groups while the main body of the solvent interacts with the polymer backbone. For example, xylene is an excellent solvent for the polystyrene backbone and when combined with 5% methanol readily and rapidly will dissolve the previous example of lightly sulfonated polystyrene.
The remarkable and surprising discovery of the instant invention pertains to the following observation.
When small (or large) amounts of water are combined and mixed under high shear solutions of ionic polymers and a free radical generating initiator dissolved in such mixed solvent systems as those described above, it is possible to convert such low viscosity systems into extremely viscous gels or solutions having high creep resistance at elevated temperatures. Indeed it is possible to achieve increases in viscosity by factors of 10.sup.4 (10,000) or more by the addition of only 5 to 15% water based on the polymer solution volume.
This unusual behavior is postulated to arise from the removal of the polar cosolvent from the polymer solution phase into a separate aqueous phase. Consequently, when this occurs, the physical crosslinking of the ionic groups is again manifested resulting in a tremendous increase in solution viscosity. The resulting gels appear quite homogeneous.